This invention is concerned with the shaping of metals by controlled removal of material from the surface of the workpiece being shaped or sized. It relates in particular to a method of improving the efficiency of some conventional metal-shaping tools by changing the tool/workpiece surface interface conditions to increase the rate at which the tool can remove metal under certain operational conditions.
A common way of shaping a metal workpiece by the removal of material therefrom involves rubbing contact, as experienced in a conventional wedge-shaped metal or ceramic cutting tool with a sharp edge (a technique generally known as "machining"). Here, the tool's cutting edge is set so it can penetrate the workpiece surface, and rubbing takes place just below the original surface level to cause material to be sheared from the surface being machined. The tool with the cutting edge can take many forms--for example, teeth on a rotating mill cutter, or a chisel-like tool in a lathe (tools of the latter type are often referred to as single point cutting tools). Alternatively, rubbing contacts can be between burnishing lands on a rotary tool, or between a polished raised ring on a linear tool (like a burnisher on a broach tool). Here the rubbing takes place only at the contact area with the surface, and material is wiped or smoothed out but generally not sheared from the surface. This method of shaping is an instance of forging, and when done cold is often referred to as cold working. Examples of the materials used in the tools used in cutting and cold working are tool steels, tungsten carbide, alumina, cubic boron nitride, and natural and artificial diamond.
Another important type of material removal used in metal shaping employs abrasive rubbing tools, typified by conventional grinding wheels. These use many very hard and small crystalline grains (or "grits") of abrasive material with a multiplicity of cutting faces (in the tool the angles of the cutting faces of these abrasive grains will be randomly distributed with respect to the machined surface). These abrasive grains range in size typically from 0.01 mm to 0.4 mm across, and are distributed at densities from about 20 mm.sup.-2 down to less than 2mm.sup.-2. They are commonly used in lapping and honing stones, grinding wheels, super-finishing stones, and the abrasive media used in tumbling or vibratory polishing and finishing processes. Examples of the abrasive materials are garnet, emery, pumice, silica, diamond, carbides of iron, or tungsten, silicon carbide, cubic boron nitride, and aluminum oxide (alumina).
In the case of conventional abrasive tools less than 50% of the grains' contacting faces are statistically at angles suitable for efficient shear cutting; the remaining angled faces cause ploughing and a good deal of smearing and burnishing by rubbing resulting in large amounts of unwanted cold working and energy dissipated as friction-generated heat. This is wasteful, and accounts in large measure for the relative inefficiency of abrasive cutting systems when compared with conventional shear cutting described above.
In conventional cutting and abrading it is commonplace to introduce at the cutter/workpiece interface a material that principally acts as a coolant and as a chip remover (to wash the cut chips away from the cutting tool) but which normally has some (and often claimed as important) lubricating properties. Conventional theory says the lubricating properties of the coolant are important to minimize rubbing friction at sites where (in abrasive machining) the grains are not orientated favorably for efficient cutting, and to minimize friction as sheared material passes across the rake face of grains or (in conventional cutting tools) of wedge shape cutters. In this latter case clean and efficient metal removal is only possible when the rake angle is favorable, allowing the cutting element to penetrate into the surface so as to transmit a force into the material being cut that is generally parallel to the surface to allow the material immediately ahead of the tool plasticly to deform and shear from the surface. If, however, the rake angle is such (leaning forward) that the tool is inclined to ride up over the surface to be cut, then rubbing and ploughing (a sideways displacement of material) occurs, which is not only wasteful of energy but in some cases causes severe surface damage as well as inducing residual surface tensile stress.